The present invention relates generally to the fields of optics and characterization of reflective surfaces. Specifically, it concerns an apparatus for determining the near-angle scattering of a sample reflective surface, and a method of determining the near angle scattering of a sample reflective surface using optical heterodyne detection.
The metallic coatings of reflective surfaces, e.g., mirrors, are known to have anisotropic structures that result in polarization dependent scattering behavior. These effects arc evident as small perturbations of the point spread function (PSF) known as near angle scattering. For purposes herein, near-angle scattering is defined as scattering determined at less then 10° from the centroid of the point spread function, also referred to herein as the specular reflection peak.
In addition to contributions from mirror coatings, small amounts of scattering from lenses, mounts, and/or other elements in an optical train may cause the specular peak to grow in angular extent. This combination of factors entirely overwhelms the ability to determine the near-angle scatter from a mirror sample using current methods and practices.
The Bidirectional Reflective or Scattering Distribution Function is currently measured through use of a scattering goniometer instrument, referred to in the art as a scatterometer. The instrumental beam width (collection angle span) over which a typical scatterometer responds is at least 10°, often greater then about 20° from the centroid of the point spread function. The difficulty of measuring scattering properties of coated mirrors extremely close to the specular reflection peak thus necessitates a new measurement approach.
Metallic mirror coatings have long been known to be mildly anisotropic due to the columnar nature of the metal deposition (i.e., the reflective coating.) See for example, J. Breckinridge and B. Oppenheimer, “Polarization Effects in Reflecting Coronagraphs for White Light Applications in Astronomy”, Astrophysical Journal 600 (Jan. 10, 2004): pp. 1091-1098.
This anisotropy and its variation over a mirror surface will, to some degree, affect the scattering properties and PSF properties of the mirror. However, the terrestrial planet finder (TPF) optical system concepts currently under development require extremely small amounts of scatter at angles very near to the specular beam in order to separate the light of bright stars from that of dim orbiting planets. This is also true in a variety of technologies including telecommunications, analytical measurements, and the like, which utilize various forms of reflective surfaces to function. Measuring the polarization dependent mirror scattering properties is important to quantify the effects of surface coatings on the reflected light, and to provide metrology technology for optimizing mirror coatings for the TPF and other instruments and applications affected by near-angle scattering.
Disclosures directed to characterizing reflective surfaces include U.S. Pat. No. 6,403,966 to Oka (Oka), which is generally directed to a measurement method and a measurement apparatus for measuring the structure of a micro-structure or the structure along the depth of an object for measurement. The laser light from a solid-state laser light source is subjected to wavelength conversion to generate the ultraviolet laser light, and measurement is made of the object for measurement by heterodyne detection or homodyne detection employing the ultraviolet laser light. This enables measurement of the micro-structure of the sample. Alternatively, the laser light is split into multiple laser light beams and frequency shifted so that the laser light beams will be of different frequencies. The laser light beams are imaged at respective different focal point positions to perform heterodyne detection. The resulting heterodyne signals are separated into respective frequency bands and measurement is made of the structure of the object for measurement in association with the respective imaging points. This enables measurement of the structure of the object for measurement in the direction along its depth. Oka is directed to an apparatus and a method in which the sample being analyzed is moved within the light beam to measure surface characteristics of the sample. However, this approach does not allow for measurement and characterization of near-angle scattering of a mirror surface.
U.S. Pat. No. 5,477,319 to Shimonaka et al. (Shimonaka) is generally directed to an optical heterodyne detection method of detecting a beat signal by heterodyne detection using a superimposition of two or more beams of light including a signal beam and a reference beam. Shimonaka discloses optical heterodyne detection of scattered light, but fails to provide any disclosure or suggestion directed to an instrument or method capable of measuring near-angle scatter of a mirror.
U.S. Pat. No. 5,883,714 to Jann et al. (Jann) is generally directed to a non-contact optical inspection instrument and method for measuring the height and width of defects and contaminants on a magnetic disk surface. The instrument includes a sensor which produces an illumination beam that is modulated and then focused normally on the disk surface as a spot. The illumination spot is Doppler shifted due to the movement of the disk and the diffusely reflected light is interfered with a reference beam produced by the sensor's illumination optics. Jann discloses utilizing scattered light in an optical detection method; however, Jann fails to provide any disclosure or suggestion directed to an instrument or method capable of measuring near-angle scatter of a mirror.
U.S. Pat. No. 6,879,402 to Kuchel, and its continuation in part U.S. Pat. No. 6,972,849; and its continuation in part U.S. Pat. No. 7,218,403 (collectively referred to as Kuchel) are generally directed to interferometric scanning method(s) and apparatus for measuring optics either having aspherical surfaces or that produce aspherical wavefronts. In the disclosures, a test optic is aligned and moved with respect to a scanning axis relative to the origin of a known spherical wavefront that is generated with a reference surface to intersect the test optic at the apex of the aspherical surface and at radial zones where the spherical wavefront and the aspheric surface possess common tangents. The test surface is imaged onto a space resolving detector to form interferograms containing phase information about the differences in optical path length between the reference surface and the test surface while the axial distance which the test optic moves relative to the spherical reference surface is interferometrically measured. Kuchel discloses an apparatus and a method in which the sample being analyzed is moved within the light beam to measure surface characteristics of the sample. However, Kuchel fails to disclose or suggest a method of measuring near-angle scattering.
U.S. Patent Publication No. 20080030742 to Hill (Hill) is generally directed to an apparatus and method for in situ and ex situ measurement of spatial impulse response of an optical system using phase-shilling point-diffraction interferometry. Hill discloses a test object including: an arrangement of optical elements defining a first partially reflecting surface and a second partially reflecting surface, at least one of the first and second partially reflecting surfaces being curved, wherein the first partially reflecting surface is arranged to receive a substantially collimated input beam and produce therefrom a first transmitted beam that passes on to the second partially reflecting surface, wherein the second partially reflecting surface is arranged to receive the first transmitted beam from the first partially reflecting surface and produce a collimated second transmitted beam and a first reflected beam therefrom, wherein the first partially reflecting surface is arranged to receive the first reflected beam and produce a second reflected beam therefrom, and wherein the first and second reflecting surfaces are configured to cause the second reflecting beam to converge onto a spot on a back surface to produce a diverging beam traveling in the same direction as the collimated output beam. However, Hill fails to disclose or suggest a method of measuring near-angle scattering.
Other references of interest include J. Breckinridge and B. Oppenheimer, “Polarization Effects in Reflecting Coronagraphs for White Light Applications in Astronomy”, Astrophysical Journal 600 (Jan 10, 2004): pp. 1091-1098; and B. Devaraj, M. Takeda, M. Kobayashi, M. Usa, and K. P. Chan, “In vivo laser computed tomographic imaging of human fingers by coherent detection imaging method using different wavelengths in near infrared region”, Appl. Phys. Lett. 69 (1996) pp. 3671-3673.
Accordingly, a method and apparatus capable of measuring near-angle scattering is a long-felt need in the art. An apparatus and a method have been discovered which are capable of measuring near-angle scattering using coherent, heterodyne detection to enable very low noise scattering measurements of reflective coatings and other surfaces.